Diabetes is a polyetiological metabolic disease, characterized by chronic hyperglycemia accompanied with disorder of glucose, fat and protein metabolism caused by insulin secretion and/or effect defects. Diabetes is also a very old disease. It is caused by relative or absolute lack of insulin in the human body, which leads to rise of the concentration of glucose in the blood and this further leads to excretion of a large volume of glucose from urine accompanied with symptoms such as polydipsia, polyuria, polyphagia, emaciation, dizziness, debilitation, etc.
In diabetes treatment, exercise therapy and diet therapy are two essential types of therapeutic methods for diabetes. When these two therapeutic methods are not sufficient to control the disease, insulin or oral hypoglycemic drugs can be used. But because there are many side effects in these hypoglycemic drugs, it is particularly important to develop a new and low-side effect drug that can effectively treat diabetes. DPP-IV is a serine protease; it can split N-terminal dipeptidase in a peptide chain containing a proline residue at the secondary end; although the physiological effect of DPP-IV to mammals has not been fully confirmed, it plays a very important role in the process of neural enzyme metabolism, T-cell activation, cancer cells metastasizing in endothelium and HIV virus entering lymphoid cells (WO98/19998).
Studies have shown that DPP-IV can inhibit the secretion of Glucagon-Like Peptide (GLP)-1 and split group-propylene peptidase at N-terminal in (GLP)-1 so that it is degraded from active form (GLP)-1 (Endocrinology, 1999, 140:5356-5363). Under physiological conditions, the half-life period of the intact (GLP)-1 is short in circulating blood, and DPP-IV inhibitor can completely protect the endogenous and exogenous (GLP)-1 from inactivating by DPP-IV, which greatly improves the physiological activity of (GLP)-1 (from 5 to 10 times). Because (GLP)-1 is an important stimulator for secretion of pancreatic insulin and can directly affect the distribution of glucose, DPP-IV inhibitor has very good effects for the treatment of non-insulin-dependent diabetes patients (U.S. Pat. No. 6,110,949).
Currently marketed DPP-IV inhibitors include sitagliptin, vildagliptin, saxagliptin, alogliptin and linagliptin and so on. Wherein linagliptin has less liver and kidney function damage. The structural formula of linagliptin is as follows:

According to the report sent by linagliptin to FDA, it was disclosed that bioavailability of linagliptin in mouse and human body was not high (CD-1 mouse, 5 mg/kg, F=18.4%; man, 5 mg/subject, F=30%). Therefore, providing a compound to replace linagliptin is a problem to be solved urgently.